Dazed and confused on spa wiring

I own a 6'x6' spa with 6 jets ( 4 small 2 large) and was formerly heated by natural gas in another city. Our home has only propane available and would be cost prohibitive to use so I am converting to an electrical heater. I Am considering buying a spa pack pre wired and plumbed with 5.5kw heater and 2 hp pump in 240 volt. Our current house has a dedicated circuit with a 30 amp breaker , #6 wire, consisting of three wires only (bare ground, white, and black) I have been told that this set up can be used by installing a 50 amp gfci in place of the 30 amp breaker in my 200 amp service panel (if all electrical equipment is 240 volt) and then not having to worry about a 4th wire for a gfci on the spa pack. I plan on having this done by a licensed electrician but would like to learn as much about it as possible before buying a spa pack and committing to having the electrical work done. I sent this same summary to the company (Spa Guts) but got a reply from a sales person who didn't have the vaguest Idea on how to answer my question. Any help would be appreciated. Wiring diagram would also be a great help. Thanks, T.T.